This invention relates to a gage used for measuring the amount of tension force necessary to unspool thread from a sewing machine bobbin.
In machine sewing, thread fed from sources above and below the material being sewn is interstitched by action of the machine, and the tension of the thread which is fed from both sources is critical in forming a proper switch. Furthermore, the amount of tension must be adjusted for sewing materials having different weights and thicknesses. Accordingly, the tension, or resistence, imparted to the thread from both sources must be adjustable. The feed mechanism of the main thread supply (that spread which is supplied from above the material) contains an adjustable drag which is calibrated, and therefore its tension can be set to a reasonable degree of certainty. However, the thread supplied to the bottom of the material is fed from a self contained spool or bobbin, containing integral tensioning means which are not calibrated. Therefore, the tension of the bobbin is not so easily adjusted.
In practice the machine operator does not adjust the bobbin tension and relies primarily on the main thread adjustment to tailor the thread supply tension to differing sewing conditions. In those instances where the bobbin tension must be adjusted, it is done so by trial and error, using the resulting stitch as the gage.
The problem arises when the bobbin tension becomes so far out of adjustment that the machine will not function at all or else it functions so poorly that the operator can no longer use the stitch as the means of gaging tension. When this happens the user is often unable to determine what the problem is and the machine must be taken to a repair shop for adjustment. The shop will normally completely clean and service the machine in addition to adjusting bobbin tension which adds to the cost of the repair, and the bobbin tension then is adjusted by the repairmen primarily by his sense of feel and his experience.